During atmospheric pressure plasma treatment, reactive species generated in the plasma diffuse through a boundary gas layer which is adsorbed at the material surface. Many of the reactive species become inactivated before reaching the surface due to their short lifetime. The efficiency of plasma treatment can be highly enhanced by simultaneous high-power ultrasonic irradiation of the treating surface, because the delivered acoustic energy can reduce the thickness of the boundary gas layer. Here surfaces of glass-fibre-reinforced polyester (GFRP) plates were treated using an atmospheric pressure dielectric barrier discharge in helium with ultrasonic irradiation, particularly for the adhesion improvement. The ultrasound was irradiated through a powered mesh electrode using a high-power gas-jet ultrasonic generator. The discharge mode changed from glow to filamentary by the ultrasonic irradiation. The surface characterizations were performed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force mictroscopy (AFM). O/C ratios at the GFRP surfaces before the treatments, after 30-s plasma treatment, and after 30-s plasma treatment with ultrasonic irradiation were 0.295, 0.385 and 0.447, respectively. This indicated that the plasma treatment oxidized and roughened the GFRP surface, and the ultrasonic irradiation further enhanced the oxidation. It is concluded that plasma treatment efficiency for adhesion improvement of GFRPs is enhanced by the ultrasonic irradiation. (C) Koninklijke Brill NV, Leiden, 2010